1. Field of the Invention
The present invention relates to a device which is to be mounted on a printed-circuit board working machine for holding or pressing down a printed-circuit board during working operation. More specifically, the invention relates to a printed-circuit board holding device which has a plurality of holding or pressing sections and changes them according to the size of a portion to be worked of a printed-circuit board.
2. Description of Related Art
In order to precisely work a printed-circuit board, it is necessary to hold or press down the printed-circuit board firmly. For example, in case of perforating the printed-circuit board by means of a drill, it has been customary to hold down the vicinity of a portion to be worked of the printed-circuit board so as to prevent production of any burr along the edge of a drilled hole.
It is convenient for the printed-circuit board working machine to have an equipment for that purpose. A conventional example of this kind of working machine is disclosed in Japanese Patent Unexamined Publication No. 4-354609.
The conventional working machine is constructed to hold a working tool or drill on a spindle and to work a printed-circuit board set on a table through driving of the spindle. A printed-circuit board holding device includes a pressure foot which is provided at an end of the spindle for relative movement thereto. The pressure foot has a through hole for allowing the drill to pass through. When working, the pressure foot is first brought into contact with the printed-circuit board to hold down the board against the table. Subsequently, the drill is passed through the through hole in the pressure foot and brought into contact with the printed-circuit board. In this way, working is performed in a state where the printed-circuit board is held down.
Typically, a plurality of kinds of holes, the sizes of which are different from each other by 0.05 mm -0.1 mm and range from 6.3 mm at the largest to 0.1 mm at the smallest, are formed in the printed-circuit board, and a plurality of sizes of drills are used in working one printed-circuit board. The through hole in the above-described pressure foot is made larger than a diameter of the largest drill so as to allow all the drills to pass through it.
Incidentally, as holding or pressing down of the workpiece is performed closer to the worked position, the effect becomes greater. In case of drilling the printed-circuit board, degree of production of burr depends on a distance between an outside diameter of the hole being drilled and the pressing position. For this reason, it is desirable to hold down the printed-circuit board at as close position to the portion to be worked as possible.
The conventional device described above, as the through hole in the pressure foot is large, has a low pressing effect when working a hole of small diameter. To cope with this, it has been proposed in Japanese Patent Unexamined Publication No. 4-354609 that through holes of various sizes are formed and changed selectively according to the size of tool to be used.
More specifically, the device according to Japanese Patent Unexamined Publication No. 4-354609 has a plurality of holding or pressing sections rotatably or slidably provided on a lower side of the pressure foot. These holding sections have through holes of different diameters, respectively. When working, the holding sections are first moved to align one of the through holes thereof, which matches the size of the tool to be used, with the through hole in the pressure foot. Subsequently, working is performed. In this way, it is possible to perform the working operation while pressing the vicinity of a hole to be formed.
In the device according to Japanese Patent Unexamined Publication No. 4-354609, during selection of a holding section prior to the working, the drill is held at a sufficient distance from the moving holding sections lest it should interfere therewith. Accordingly, the drill must be moved through this distance until it is brought into contact with the printed-circuit board, with the result that the stroke required for the formation of one hole is made large.